The Rock from Mars: A Detective Story on Two Planets
By Kathy Sawyer
Discovering life beyond Earth will change forever our view of the Universe and our place within it. So when a team led by David McKay of NASA's Johnson Space Center announced in August 1996 that they had found evidence for ancient life in a martian meteorite (D. S. McKay et al. Science 273, 924–930; 1996), the world sat up and took notice. How that announcement came to pass and what happened afterwards form the subject matter of The Rock From Mars.
Kathy Sawyer, a journalist formerly with The Washington Post, has produced a model of science writing for the general public. She gets the science right (with an occasional bobble), she reports with commendable balance on the intense controversy generated by McKay and colleagues' announcement, and her lucid writing should prove largely comprehensible to non-scientists. She accurately depicts the day-to-day life of the scientists involved and brings out how their observations and ideas are processed by the scientific community as it gropes its way towards the truth.
McKay's team got one thing dead right: if you want the scientific community to believe you've found evidence for extraterrestrial life, you should provide several independent lines of evidence, something Sawyer terms a ‘holistic’ approach. The lines of evidence they use are termed ‘biosignatures’. The critical feature of a biosignature is not that it can be produced by biological activity, but that it cannot be produced non-biologically. Evaluation of McKay and colleagues' claim therefore rested on whether their lines of evidence were really biosignatures, or whether they could have been produced non-biologically.
Their observations of meteorite ALH84001 (which is firmly established as martian even though it was found in Antarctica) led McKay and colleagues to advance four potential biosignatures. Two of them — the presence of organic matter and ultra-small fossil-like forms — were quickly rebuffed. The organic matter, although partly of apparently martian origin (itself a notable achievement by the team), consisted of polycyclic aromatic hydrocarbons, which are not plausible biosignatures as they can easily be made by non-biological processes. The ‘fossils’ could not be reliably distinguished from textures intrinsic to their mineral hosts, which had not previously been studied at such high magnification. That left two candidate biosignatures: unusual layered globules of carbonate, and, contained within them, tiny magnetite crystals that closely resembled those in magnetotactic bacteria on Earth.
Towards the end of chapter 13, Sawyer cites several studies that “soon challenged the biological scenario anew”. But then the chapter winds down without further reference to these studies, two of which turn out to be particularly important (they are briefly described in the notes). In the final two chapters, Sawyer shifts gears twice, once to discuss a marginally related controversy, and finally to report on some recent missions to Mars. Both chapters would have made interesting appendices to the book, but the claim made by McKay and colleagues is left hanging in limbo; the “detective story” ends not with a bang but a whimper. This is particularly unfortunate as the two studies mentioned above would have provided the bang of two smoking guns.
First, a team from the Johnson Space Center led by Gordon McKay, David's brother, succeeded in synthesizing identical layered carbonate globules in the absence of biological activity, thereby destroying the globules' status as a plausible biosignature (D. C. Golden et al. Am. Mineral. 86, 370–375; 2001). Second, a study by David Barber and Ed Scott (Proc. Natl Acad. Sci. USA 99, 6556–6561; 2002) drove the final nail into the coffin. They showed that the crystal lattices of the ultrafine magnetites within the meteorite's carbonates were topotactically related to that of the carbonate host, a point that I had made in 1999, based on findings by John Bradley and co-workers (Meteorit. Planet. Sci. 33, 765–774; 1998). Note that this is a true three-dimensional relationship, as first observed synthetically by J. D. Bernal and co-workers (Clay Mineral. Bull. 4, 15–30; 1959); it should not be confused with epitaxy, a two-dimensional relationship. It means that the magnetites must have formed directly by decomposition of carbonate, and could not plausibly be bacterial in origin. Barring an extraordinary coincidence, this conclusion must apply to all the nanoscale magnetites in the carbonate.
Even though I would have liked the book to conclude with closure of this tumultuous episode, this is nonetheless an outstanding popularization of science that deserves to be widely read, not least by those interested in the ‘logic of scientific discovery’.
Finally, it is worth noting that, despite the demise of the biosignatures proposed by McKay and co-workers, the search for evidence of ancient life on Mars remains scientifically reasonable and, indeed, of fundamental importance.